A presidential debate in 3 parts

A stagehand uses a lint roller to clean the background on the stage at the Magness Arena at the University of Denver, Tuesday, Oct. 2, 2012, where the first presidential debate between President Barack Obama and Republican presidential candidate Mitt Romney is scheduled for Oct. 3. (AP Photo/Ed Andrieski)

WASHINGTON (AP) -- There's more to Wednesday night's presidential debate than just the 90 minutes onstage. For the campaigns, it's a three-part performance:

Part I: Aw-shucks time

Setting low expectations can help a so-so performance seem like a success.

So in the days before their first meeting, President Barack Obama called Republican challenger Mitt Romney "a good debater" and deemed his own skills "just OK." His aides groused that Romney got more rehearsal time, while Obama was busy being president.

For his part, Romney praised Obama as "a very eloquent, gifted speaker." And, despite his numerous GOP primary match-ups, Romney noted, "I've never been in a presidential debate like this."

Part II: Tension city

The first of the three presidential debates - starting at 9 p.m. EDT in Denver - should bring the biggest audience of any campaign event. More than 52 million TV viewers watched Obama's initial match-up with John McCain in 2008.

Despite all the rehearsal, something's bound to take the candidates by surprise, and they'll be judged by how they improvise on the fly. Talk about "tension city," as former President George H.W. Bush described it.

But maybe Romney and Obama should each take a deep breath. After all, how likely is it that either one will commit a big enough blunder to overshadow months of campaigning? Studies find viewers tend to see the guy they preferred going into the debate as the winner when it's over.

"When is it that anybody performs so badly that you'd just say, `Oh, my God, I would never vote for this person'?" said Rutgers University professor Richard Lau, who studies how voters decide. "Someone would have to seem so incompetent. That's not going to happen."

Part III: The spin

It's not over when the candidates walk off stage.

Campaign aides and big political names will descend on the "spin room" to tell reporters and after-debate TV audiences that the other guy blew it, and why.

Viewers may feel they're judging what they saw and heard for themselves. But campaign strategists think getting the spin right goes a long way toward deciding who "won."

According to Tad Devine, who was a top adviser to Democratic candidates Al Gore and John Kerry, pre-debate expectations and post-debate spin "can take on more significance than what happened in the debate itself."